Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2019	Dec. 31, 2013	Jan. 01, 2019
Summary Of Significant Accounting Policies [Line Items]
Maturity period of highly liquid debt instruments		Three months or less
Available-for-sale securities transferred to held-to-maturity securities	$ 536,000,000.0		$ 536,000,000.0
Unrealized net loss on securities reclassified from available-for-sale to held-to-maturity, Gross	$ 13,300,000		$ 13,300,000
Loans past due, minimum period		90 days
Non-accrual business and commercial real estate loans		$ 250,000
Delinquency period of losses on mortgage loan to be deemed uncollectible		120 days
Allowance for loan losses		$ 0
Mortgage loans sold on real estate, final maturity period		15 years
Operating lease, right-of-use asset		$ 18,682,000
Operating lease liability		$ 18,893,000
Computer Equipment
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets (years)		3 years
Furniture, Fixtures and Other Electronic Equipment
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets (years)		5 years
Building Improvements
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets (years)		10 years
Buildings
Summary Of Significant Accounting Policies [Line Items]
Estimated depreciable life of assets (years)		30 years
Other assets | Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease, right-of-use asset				$ 20,600,000
Other liabilities | Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Operating lease liability				$ 20,700,000